Thaw fish, if frozen. For sauce, in a small saucepan combine jam, onion, thyme, mustard, and garlic. Bring just to boiling, stirring frequently; reduce heat. Simmer, uncovered, about 10 minutes or until sauce is slightly thickened. Remove from heat. Cover and keep warm.
Rinse fish; pat dry with paper towels. Coat the unheated rack of a broiler pan with cooking spray. Place fish on the prepared rack. Brush with about half of the melted margarine. Sprinkle with salt and, if desired, black pepper.